The olfactory receptor genes have been characterized through homology as seven transmembrane domain G protein-coupled receptors (GPCR). It is estimated that there are probably 500-750 olfactory receptor gene sequences in humans, while there are 500-1000 olfactory genes in rat and mouse. Olfactory receptors are concentrated on the surface of the mucus coated cilia and odorant molecules bind to the olfactory receptors in the olfactory epithelium. Since mammals can detect at least 10,000 odors and there are approximately 1,000 or fewer olfactory receptors, many odorants must interact with multiple olfactory receptors.
The discriminatory power of olfactory receptors is such that it can perceive thousands of volatile chemicals as having different odors. It is known that the olfactory system uses a combinatorial receptor coding scheme to decipher the odor molecules. One olfactory receptor can recognize multiple odorants and one odorant is recognized by multiple olfactory receptors. A slight structural change in the odorant or a change in the concentration of the odorant in the environment results in a change in the odor-code of these receptors.
Odor molecules belong to a variety of chemical classes: from alcohols, aldehydes, ketones and carboxylic acids to sulphur-containing compounds and essential oils. The physicochemical descriptors of odor molecules play an important role in the prediction of odor response by the olfactory receptor. Similar olfactory receptor sequences can have a structural bias for ligand specificity on the basis of the number of carbon atoms present in the ligands. About 8000 odorants have been identified in food. About 400 food odorants have been characterized and this number approximately equals the number of olfactory receptors found in humans. The response of mixtures of odorants is neither the additive nor an average of its components. Some mixtures lead to the emergence of novel perceptual qualities that were not present in the individual components.
Olfactory receptors have been found in non-olfactory tissues such as, for example, adipose tissue, adrenal glands, brain, breast, colon, white blood cells, the gut, heart, kidney, liver, lung, lymph nodes, ovary, placenta, prostate, skeletal muscle, testis and thyroid. ORs expressed in these non-olfactory cells can be referred to as ectopic ORs. In olfactory neurons, only one allele of an OR gene is expressed per neuron. In contrast, non-olfactory cells multiple OR genes can be expressed in each cell. Some ectopic ORs are evolutionarily conserved across mammals, and several ectopic ORs are broadly expressed across many different cell types in mammals.